The present invention relates to an optical disc apparatus used, for example, as external memory means of a computer, and more particularly to a positioning system of an optical disc apparatus for adequately positioning a light spot relative to an information recording surface of an optical disc.
An example of this type of a positioning control system of a conventional optical disc apparatus is disclosed in JP-A-6-203390. In this prior art, the focus servo system and the tracking servo system are configured so that arithmetic operations for control are divided and executed by two separate circuits, that is, an analog circuit is used to execute those operations requiring high speed processing are performed by and a digital circuit is used to execute those operations which are complicated but may be processed at low speed.
Generally, there is a shift of the center of a focus error signal and a tracking error signal from the electrical center of a drive signal owing to a shift of the optical axis of an objective lens and an electrical offset of an amplifier, for example. To eliminate this shift, it is required to make a correction to bring the center of the focus error signal and the tracking error signal into coincidence with the electrical center of the drive signal.
With regard to the above-mentioned elimination of the shift, heretofore a method has been used in which a value corresponding to an amount of shift from the electrical center of the drive signal is obtained empirically, and the obtained value is added to the drive signal.
In the positioning system of the conventional optical disc apparatus mentioned above, to respond to the strong demand for a functional improvement of the servo system, introducing digital control is effective indeed. In parallel with this demand, there has been a growing demand for speeding up of data transmission and it has become necessary to increase the rotation speed of the optical disc. Under this situation, a problem has arisen as described below.
As the optical disc is rotated at higher speed, the frequencies of disturbances such as vertical deflection and eccentricity of disc surface in synchronism with the rotation of the optical disc will increase. In other words, it is necessary to provide notably improved disturbance suppression characteristics.
As a conventional technique to increase the disturbance suppression characteristics, there is a method in which arithmetic means requiring stability is formed as an analog circuit, and arithmetic means to obtain greater gain at low frequencies is formed as a digital circuit, and the coefficient of the digital arithmetic means is set at an optimum value according to the control condition.
However, in a conventional servo system in which the arithmetic means required to have stability is formed by an analog circuit as mentioned above, when the gain of an actuator or the like differs from a standard value due to an individual difference of the apparatus, it is necessary to make adjustment to secure stability.
Further, in order to set a coefficient of the digital arithmetic means according to the control condition, a large memory is required to store a plurality of coefficients. In addition, the coexistence of an analog circuit and a digital circuit makes the circuit configuration large and complicated.
On the other hand, in the prior art mentioned above, in the method of correcting the shift of the center of the focus error signal and the tracking error signal from the electrical center of the drive signal, a value corresponding to the amount of shift from the electrical center of the drive signal is obtained empirically, and the obtained value is added to the drive signal. In this method, however, if a shift occurs due to an individual difference of the apparatus, adjustment is required.